Lazy approval infinite emails

Hello great community people,

Has any of you seen this before?!

A user had accidentally responded to an email from SharePoint about a specific task. The email didn't include the correct outcome. SharePoint sent back the regular response saying that the response wasn't able to be interpreted.

The suddenly duplicate emails are being send to the user every 30 seconds with the bold line below..

We have terminated the workflow, deleted the item and made sure the list item even is no longer there. We purged the workflow data. Nothing seem to stop those infinite notifications.

First: How can we stop those notifications from being sent to the user?

Second: What could have caused this to happen?

Third: How would we provent this from happening if ever one user accidently responded to an email from SharePoint with an Invalid outcome!